1. Field of the Invention
The present invention relates generally to secondary power sources and, more particularly, is concerned with a water-cooled AC and DC motor-generator set employing AC and DC side rotors in a tandem arrangement on a common shaft with a series cooling flow path defined through the shaft and rotors.
2. Description of the Prior Art
A secondary power source is typically provided in a nuclear submarine to generate reserve power. As the primary power source of the submarine, its nuclear power plant, is operated to power the submarine, it also powers the secondary power source to charge DC batteries. Then, during periods when the nuclear plant is shut down or knocked off-line, the reserve power of the DC batteries is used to restart the primary source to operate the submarine.
One typical secondary power source used heretofore employs an AC and DC motor-generator set in which the AC and DC components are disposed in tandem arrangement on and along a common solid shaft. During periods of nuclear power plant operation, the AC component functions as a motor to rotatably drive the common shaft and thereby run the DC component as a generator to charge the DC batteries. Then, during periods when the nuclear power plant is shut down, the DC batteries operates the DC component as a motor to rotatably drive the common shaft and thereby run the AC component as a generator to operate the submarine.
Historically, the AC and DC components were cooled by air flow about the peripheries thereof. Air cooled systems are noisey due to the vane turbulance of their fans. Further, air cooling can draw silicon resin vapors onto the electrical contact brushes which reduces their useful life. And still further, with the new higher power densities demanded of these components, more heat is generated and they are noisier. In view of the criticality of and need to conserve space and weight in a submarine as well as for the submarine to operate quietly, a primary objective in the design of its components, which includes the components of the motor-generator set, must be to minimize their size and weight. It is perceived that the adoption of a way to cool the motor-generator set by water as opposed to air might offer opportunities to further reduce its size and weight over what has been achieved heretofore and also to operate more quietly.
Generally, internal circulation of liquid coolant through rotor shafts of turbine generators and motors to cool the rotor shaft and windings is well known. Representative of the prior art are the liquid coolant flow arrangements disclosed in U.S. Pat. Nos. to Willyoung et al (3,056,055), Krastchew (3,240,967), Stark et al (3,475,631), Heard et al (3,476,961, Guthan (3,519,861), Curtis et al (3,733,502); 3,740,596; and 3,868,520), Heller et al (3,742,266), Albaric et al (4,114,059), Linscott, Jr. (4,203,044) and Okamoto et al (4,358,937). While many liquid coolant flow arrangements of the prior art, such as those of the aforecited patents, would appear to operate reasonably well and generally achieve their objectives under the limited range of operating conditions for which they were designed, most seem to embody shortcomings which make them less than an optimum arrangement for water coolant flow through a motor-generator set in a nuclear submarine. Some arrangements provide both inlet and discharge of the coolant flow at the same end of the shaft which requires additional means to physically separate flow paths through the entire length of the shaft and introduces the possibility of undesirable heat transfer between the flow paths that would decrease the overall thermal efficiency of the cooling system. In addition, the possibility is raised of trapping air at the closed end of the shaft during the initial filling of the cooling system. Also, certain arrangements require circulation of a large volume of liquid within the rotating components which during high-speed rotational operation thereof may result in the occurrence of unpredictable vortices that will disrupt the desired flow patterns. Finally, many prior art arrangements require a large number of parts which reduce reliability and increase manufacturing and assembling costs.
Consequently, a need exists for a design which provides coolant flow through the motor-generator set in a manner which avoids the aforementioned shortcomings without introducing others in their place.